How is sine of 90 degree possible

I don't know how can a triangle be formed with two angles each 90 degree. If it is not possible, then how do we calculate sin 90 degree = 1 ?

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What is the value of sin 90? - Quora

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sin 90° = 1 (Why & How?)

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Why does sin 90 degrees equal 1? We know that formula for sin is perpendicular upon hypotenuse where hypotenuse is longest side? - EduRev Class 10 Question

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